Keep Moisture out of Walls To Avoid Rotting

By James Dulley

January 16, 2020 4 min read

Dear James: I am in the planning stage for a new home. My previous stucco home had serious damage for rotting lumber. What is the best way to avoid the same problems in the new home I am building? — Lynn R.

Dear Lynn: By far, the majority of problems with rotting lumber in a house can be traced back to excessive moisture and persistent dampness in the structural lumber and materials. Insect infestation is another problem, but it is also often related to moisture problems; insects are more likely to attack damp lumber.

The most serious problems with rotting are related to homes finished with synthetic stucco. Real cement-based stucco is permeable to water vapor, so moisture that gets inside the walls works its way outdoors. This allows the structural lumber to dry out before rot occurs.

Synthetic stucco is an excellent exterior wall finishing material. If it is installed properly so moisture entry points are sealed, moisture problems are rare. The newest guidelines for using synthetic stucco also provide a path for moisture to escape the wall cavities.

No matter what type of exterior finishing material you select for your new home, it is wise to first install a moisture barrier. Asphalt-saturated felt was used for many years, and it is still an effective material. Many homes built within the past 30 years or so do not have any type of moisture barrier.

Talk to your builder about using felt. It is not difficult to install felt, but it does take some time because it is available only in narrow rolls. Starting at the bottom of the walls, wrap the felt horizontally around the house, overlapping the edges by about 3 inches.

The most common material used today is synthetic moisture barrier film. It is very fast to install because the 10-foot-wide rolls cover the entire wall in one pass. The new films are more expensive than felt, but when you factor in the lower labor costs for the quick installation, the overall costs are probably about the same.

You should be able to find felt or moisture barrier films at most building supply outlets and home centers. If you have a problem finding it, contact the following companies for local outlets: Dupont Tyvek, (800) 448-9835; Johns Manville, (800) 654-3103; Owens Corning, (800) 438-7465; Pemko Manufacturing, (800) 283-9988; and Protecto Wrap, (800) 759-9727.

In addition to blocking moisture from entering the walls, the film also functions well as an air barrier. This reduces air leakage into the wall and, thus, into the living space. By reducing this air leakage (called infiltration), your utility bills will be lower and indoor air quality can be better controlled.

Even though the moisture barrier film blocks liquid water leaks from getting inside the wall and windows, it does allow some water vapor to pass through it. This is particularly important in climates that are very cold (or very hot and therefore use heavy air conditioning), where the water vapor, if trapped, can condense inside the walls, become moisture and saturate the lumber.

I recommend using a special tape to seal all the joints in the film. Seal it to the sheathing around all window and door openings because these are common areas for moisture to enter a house. The total cost to wrap and seal a typical house should be in the $400 range.

Send your questions to Here's How, 6906 Royalgreen Dr., Cincinnati, OH 45244 or visit To find out more about James Dulley and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at

Photo credit: psaudio at Pixabay

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